The present invention relates, in general, to an amplifier circuit apparatus of a type used to suppress electromagnetic interference and protect circuits during current discharge conditions. The present invention also relates to a method of suppressing electromagnetic interference and reducing current discharge conditions.
It is well known that some circuit manufacturers must regulate electromagnetic interference (EMI) generated during the operation of electronic devices and in particular EMI generated by electronic devices within integrated circuits (ICs). EMI is a noise condition and in the field of ICs designed to handle multi-level signals (e.g. a binary signal), the primary source of EMI is associated with the edge rise and fall time of the digital signal as it switches between the binary levels. The steep edges and sharp corners of a digital signal correspond to high frequency energy for which regulatory requirements for electromagnetic compatibility (EMC) are hardest to meet. Prior art circuits smooth the edges of the digital signal once output from the IC by slowing the rise and fall time of the digital signal output using filter components, thereby limiting the spectral range of the output signal and reducing the EMI at the output stage. However, the use of the filter components does not constitute an optimum solution; the filter components may affect performance characteristics of a laser device being driven in an undesired way.
It is also sometimes necessary for circuit manufacturers to protect electronic devices that are susceptible to damage or degradation from electrostatic discharge (ESD). In particular, an interference event known as Charged Device Model (CDM) ElectroStatic Discharge (ESD) may occur when there is a very fast discharge to a load from a high charge device such as a current driver for a laser diode. CDM ESD events are often very difficult events to protect against and industry standards for CDM ESD events are as severe as a 10 amp surge current spike with respective 2 ns rise and fall edges. The passage of a charge spike through an ElectroStatic-Discharge-Sensitive (ESDS) device can result in failure or performance degradation of the ESDS device such as punch-through of a transistor. CDM ESD is particularly prevalent amongst ICs and is not precluded from occurring in a circuit board. Factors contributing to the susceptibility to CDM ESD within ICs are the combination and positioning on the IC of the devices and how the routing metallisation is arranged between them. Prior art circuits employ a series resistor and a number of reverse biased diodes located at the output stage of the IC to negate the effect of CDM ESD. In this respect, FIG. 1 is a schematic diagram of an exemplary prior art circuit for an IC 18. The exemplary prior art circuit 18 comprises a voltage supply rail 26 to provide a supply voltage of Vcc volts. The voltage supply rail 26 is coupled to a cathode of a first diode 22, an anode of the first diode 22 being coupled to a node 23. The node 23 is coupled to a first terminal of a resistor 21, the second terminal of the resistor 21 being coupled to a bond pad 20. The bond pad 20 is electrically connected through bond wires and a chip package (not shown) to the outside world, thereby forming a route whereby the ESD event can enter the IC 18. The node 23 is also coupled to a node 29, the node 29 being coupled to an internal circuit (not shown) of the IC 18 and a cathode of a second diode 24, the second diode 24 being in series connection with the first diode 22. An anode of the second diode 24 is coupled to a ground terminal 30.
In operation, the prior art circuit 18 limits the effect of CDM ESD on the laser driver circuit through the use of the first and second reverse biased diodes 22, 24. Should a positive ESD spike above Vcc be introduced to the circuit 18 via the bond pad 20, the first diode 22 will conduct and potentially harmful ESD is discharged from the circuit 18. Similarly, conduction by the second diode 24 discharges a negative ESD spike. However, CDM ESD events are typically too fast for the first and second ESD diodes 22, 24 to respond to and what protection that remains is due to the current limiting effects of the resistor 21 alone.
According to a first aspect of the present invention, there is provided an amplifier circuit apparatus for driving a laser device, the apparatus comprising a multistage amplifier including an output stage, characterised in that means for band limiting a signal are coupled to the multistage amplifier prior to the output stage.
Preferably, the multistage amplifier further includes an input stage having an input, the means for band limiting the signal being coupled to the input of the input stage.
Preferably, the multistage amplifier includes an intermediate stage having an output, the means for band limiting the signal being coupled to the output of the intermediate stage.
Preferably, the means for band limiting the signal is a filter. More preferably, the means for band limiting the signal is an RC circuit. Very preferably, the RC circuit comprises a resistance, the resistance being series coupled.
According to a second aspect of the present invention, there is provided a use of a band limiting circuit to band limit a signal prior to an output stage of a multistage amplifier.
According to a third aspect of the present invention, there is provided a method of EMI suppression in a multistage amplifier including an output stage, the method comprising the step of: receiving a data signal for amplification by the multistage amplifier, and band limiting the data signal or an amplified version of the data signal prior to final amplification of the amplification of the data signal by the output stage.
According to a fourth aspect of the present invention, there is provided a level shifting amplifier circuit apparatus comprising a node for providing a supply voltage; an amplifier circuit capable of generating an output signal in response to an input signal, the output signal including a DC voltage bias level; characterized by DC level shift means coupled between the node and the amplifier circuit, so as to generate, when in use, a potential difference across the DC level shift means, the DC voltage bias level corresponding to a difference between the supply voltage and the potential difference.
Preferably, the DC level shift means is a resistance in combination with a current source.
According to a fifth aspect of the present invention, there is provided a use of a resistance and a current source in combination to DC level shift a supply voltage applied across an amplifier circuit.
It is thus possible to provide EMI suppression using the above apparatus and/or method employing RC filter circuits. In addition to EMI suppression, the series coupling of the resistances of the RC filter circuits provides ESD protection.